Project 2 (Bers): Abstract Project 2 focuses on understanding Na- and Ca-dependent myocyte mechanisms contributing to cardiac dysfunction and arrhythmias in heart failure (HF). Hallmarks of Na & Ca dysfunction in HF include elevated diastolic and late Na current (INaL), increased diastolic SR Ca leak, and Na/Ca exchange. These factors reduce systolic and diastolic function and promote triggered arrhythmias in HF. Ca-Calmodulin dependent protein kinase (CaMKII) is also chronically active in HF and directly promotes INaL and diastolic SR Ca leak via the ryanodine receptor (RyR) and is a lynchpin in a newly appreciated vicious cycle where elevated INaL or SR Ca leak (that cause arrhythmias) promote CaMKII activation to further promote higher INaL and SR Ca leak that leads to both contractile dysfunction and arrhythmogenesis in HF. Our central aim is to test this working hypothesis in adult failing hearts (and computer models of human hearts) and identify how breaking the vicious cycle at different points can be functionally beneficial. Aim 1 will test in rabbit ventricular myocytes if functional and arrhythmogenic effects of overload-induced HF can be prevented by inhibiting CaMKII, INaL or RyR. Aim 2 extends these tests to the intact rabbit heart level using optical mapping of [Ca]i and voltage Vm (where whole heart arrhythmias are complicated by cell-cell coupling, source-sink mismatch and conduction/reentry issues). Aim 3 will enhance and validate computational rabbit ventricular myocyte & tissue models and extend both to human (using rabbit data from Aim 1 & 2 and incorporating patient-specific iPSC-derived myocyte and clinical data (from Projects 1 & 3). These 3 Aims will provide valuable mechanistic insight into the vicious feedback signaling, and how targeting INa, CaMKII or RyR may have benefits in acquired or inherited (Project 1) HF.